[llvm] [Docs] [Developer Policy] Document best practice of not tagging a username in commit messages and PR descriptions (PR #164328)

[David Justo via llvm-commits]

davidmrdavid wrote:

SInce I made another push (although the diff has not changed). @kuhar can you help me with a final re-approval? I'll merge shortly after.

https://github.com/llvm/llvm-project/pull/164328